The Hydrophobic Selectivity

The hydrophobic selectivity is the methylene group selectivity of the stationary phase, that is, the selectivity of two compounds differing by a methylene group, which are retained predominantly due to their hydrophobicity. This selectivity is to be used, for example, in the case of neutral, hydrophobic analytes for the comparison of columns and assessment of their similarity, and has a much higher significance than the hydrophobicity treated in Section 4.4.1, and later sections. 

This is the so-called silanophilic activity of the reversed phase. The compounds affine to silanol-groups are usually bases that interact with the acidic silanol groups via ionic interactions. These interactions are energetically much stronger than the usual hydrophobic interactions of reversed phases, which usually lead to wider elution bands for basic analytes on classical reversed phases. 

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Figure 5. A schematic representation of the hydrophobic selection operated by lipid vesicles towards a library of peptides. The hydrophobic peptide(s) is selected out (bound) and in the presence of a membrane-bound condensing agent, can be polymerized on the membrane surface.

Ion-pair HPLC mode is a superposition of two competitive processes ion-exchange and reversed-phase. Component retention is strongly dependent on the type of ionpairing agent, its concentration, and most of all, on the history of the used column. The virgin reversed-phase (RP) column does show the hydrophobic selectivity in the ion-pair mode. However, with time, the adsorbent surface can become covered with a dense layer of adsorbed surfactant. This may irreversibly transform the RP column into an ion-exchange one. 

Analyte pairs that reveal the similarity of the phases with respect to selectivity for the current issues should be chosen. Assume that in one case the separation of both simple neutral molecules as well as aromatics is relevant see Figure 4.10. Ethylbenzene/fluorenone is a measure for the hydrophobic selectivity and chrysene/perylene for the aromatic selectivity. XTerra MS and AQUA as quite hydrophobic phases show good hydrophobic selectivity. 

Undoubtedly, apolar is an flexible term, especially if two analytes are involved, which is true in the case of separation factors. We use in our work a total of six quite different pairs of analytes for the investigation of the hydrophobic selectivity of phases. Without going into too much detail, after a lot of measurements and chemometric analysis, it was established that the representation in Figs. 15, 2b and 16 represents the selectivity of phases quite well. Here, some commercially available phases are listed according to decreasing hydrophobic selectivity, or polar selectivity, respectively (Fig. 16). 

The most common hydrophobic adsorbents are activated carbon and siUcahte. The latter is of particular interest since the affinity for water is very low indeed the heat of adsorption is even smaller than the latent heat of vaporization (3). It seems clear that the channel stmcture of siUcahte must inhibit the hydrogen bonding between occluded water molecules, thus enhancing the hydrophobic nature of the adsorbent. As a result, siUcahte has some potential as a selective adsorbent for the separation of alcohols and other organics from dilute aqueous solutions (4). 

Ideally, to ensure the complete removal of the metal ions from the aqueous phase, the complexant and the metal complex should remain in the hydrophobic phase. Thus, the challenges for separations include the identification of extractants that quantitatively partition into the IL phase and can still readily complex target metal ions, and also the identification of conditions under which specific metal ion species can be selectively extracted from aqueous streams containing inorganic complexing ions. 

Macrocyclic ligands such as crown ethers have been widely used for metal ion extraction, the basis for metal ion selectivity being the structure and cavity size of the crown ether. The hydrophobicity of the ligand can be adjusted by attachment of alkyl or aromatic ligands to the crown. Impressive results have been obtained with dicyclohexano-18-crown-6 as an extractant for Sr in [RMIM][(CF3S02)2N] IL/aque-... 

Hydrophobicity represented by AG° for the transfer of solute from the pure liquid to aqueous solution increases progressively with increasing temperature34>. There is, however, an extremum in the temperature—selectivity plot in some cases (e.g., R2 = i-CsHn, Ph, and p-MeC6H4) l4b,18). it appears that the observed selectivity cannot be explained in terms of hydrophobic interaction. 

Hydrophobicity plots of AQPs indicated that these proteins consist of six transmembrane a-helices (Hl-H6 in Fig. la) connected by five connecting loops (A-E), and flanked by cytosolic N- and C-termini. The second half of the molecule is an evolutionary duplicate and inverse orientation of the first half of the molecule. Loops B and E of the channel bend into the membrane with an a-helical conformation (HB, HE in Fig. lb) and meet and each other at their so-called Asn-Pro-Ala (NPA) boxes. These NPA motifs are the hallmark of AQPs and form the actual selective pore of the channel, as at this location, the diameter is of that of a water molecule (3 A Fig. la and b). Based on the narrowing of the channel from both membrane sides to this small... 

Fig. 32. 2H spectra of a polymer model membrane, cf. Fig. 27b), selectively deuterated at the a-methylene group of the hydrophobic chain. The spectra are compared for the monomer as well as the polymer lamellar phases at the same temperatures, respectively... 

Phosphoric acid esters based on alkylene oxide adducts are of great interest. Their properties can be altered by the length and structure of the hydrophobic alkyl chain. But they are also controlled by the kind and length of the hydrophilic alkyleneoxide chain. The latter can easily be tailored by selection between ethylene oxide and propylene oxide and by the degree of alkoxylation. 

Importantly, there was a general marked selectivity for inhibition of influenza A over influenza B viral sialidases in the carboxamide series (e.g. as seen with 27) (Smith et al. 1996, 1998), determined from crystallographic and molecular modelling studies (Smith et al. 1996 Taylor et al. 1998) to be due to the relative abilities of each of the sialidases to absorb the structural changes required to accommodate the hydrophobic alkyl chains in the glycerol side-chain binding pocket. In influenza... 

The majority of NNRTIs share common conformational properties and structural features that allow them to fit into an asymmetric, hydrophobic pocket about 10 A away from the catalytic site of the HlV-1 RT, where they act as non-competitive inhibitors (Kohlstaedt et al. 1992). However, the NNRTIs select for mutant virus strains with several degrees of dmg resistance. 

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